Devices or optical systems of this particular kind are commonly used to project electromagnetic radiation, especially visible light. Such a device, for example, may illuminate a desired area or footprint, to allow people to see freely or observe specific objects within the desired area. Other illumination devices project radiation over a hemisphere, i.e., 2.pi. steradians, for a variety of applications. Examples of these later applications include use as a warning light on an emergency vehicle's light bar or as an aircraft warning light. In each different application, the projected light has a predetermined intensity distribution to facilitate the intended or expected use of the light. To illuminate a surface, such as a picture displayed on a wall, a desktop or a floor or sidewalk, the intensity distribution may need at least some minimum intensity over the entire desired surface area. In such applications, producing the requisite intensity on the edges of the area has often required excess illumination in the center of the area. Other applications require a uniform intensity profile over a hemispherical field of view.
One type of apparatus intended to provide uniformity with respect to angle is a translusive diffuser, which includes a sheet of translusive material disposed over a light source. Light from the light source passes through the translusive sheet and is diffused into the surrounding space. Suitable translusive materials include opal glass, acrylics, and Teflon. The translusive sheet sometimes is covered by a transparent protective dome or the sheet, itself, sometimes is formed into a dome shape. Although generally effective in projecting light substantially uniformly, translusive diffusers are considered to be unduly inefficient, projecting only about 20% to 25% of the available light when configured as a hemispherical light distributor.
Another type of apparatus of this kind is a beam redirector, which includes an optical device having a special refractive or reflective surface for redirecting light from a light source such as a laser into a hemisphere. Although such beam redirectors are relatively inexpensive to manufacture, they typically require a critical alignment between the light source and the optical device, and they typically require the use of a light source that provides a light beam having a known, stable intensity distribution. Moreover, the projected light often has a spatial intensity distribution that varies excessively.
Lighting systems for interior rooms or for streets or sidewalks and the like have used a wide variety of different types of diffusing mechanisms. Many lights have a translucent globe around the source. Other lighting devices use reflectors with diffusers over the reflectors and/or baffles at the opening or within the reflectors. These various mechanisms help to distribute radiation in some desired patterns, but only to a limited extent. Also, such mechanisms tend to reduce the efficiency of the illumination of radiation from the lighting system. Many techniques for distributing the energy also tend to be unduly complex, increasing manufacturing costs and/or resulting in systems that are easily damaged.
It should, therefore, be appreciated that there is a need for an apparatus that projects electromagnetic radiation over a predetermined field of illumination with a tailored spatial intensity distribution, wherein the radiation is projected with greater efficiency and with less dependence on a critical alignment of optical components. In many cases, the tailoring needed relates to a uniform intensity distribution, over a surface area or over some range of angles defining the field of illumination. The present invention fulfills these needs.